1. Technical Field
This invention relates generally to the manufacture of integrated circuit (IC) chips, and more specifically, to a photoresist material that includes both positive tone and negative tone attributes.
2. Background Art
Manufacturing of semiconductor devices is dependent upon the accurate replication of computer aided design (CAD) generated patterns onto the surface of a device substrate. The replication process is typically performed using lithographic processes followed by a variety of subtractive (etch) and additive (deposition) processes.
Photolithography, a type of lithographic process, is used in the manufacturing of semiconductor devices, integrated optics, and photomasks. The process basically comprises: applying a layer of a material that will react when exposed to light, known as a photoresist or, simply, a resist; selectively exposing portions of the photoresist to light or other ionizing radiation, i.e., ultraviolet, electron beams, X-rays, etc., thereby changing the solubility of portions of the material; and developing the resist by washing it with a basic developer solution, such as tetramethylammonium hydroxide (TMAH), thereby removing the non-irradiated (in a negative resist) or irradiated (in a positive resist) portions of the layer.
Conventional positive and negative tone photoresists are characterized by a dissolution curve in which there is a single transition from a first dissolution rate to a second dissolution rate as the resist is exposed to varying levels of actinic radiation. In a positive resist, the initially unexposed resist is insoluble in developer, while the exposed resist becomes more soluble as the exposure dose is increased above a threshold value. For a negative resist, similar behavior is observed, except that the initially unexposed resist is soluble in developer, and the exposed area is rendered insoluble. By means of this differential solubility between the exposed and unexposed resist areas, it is possible to form a pattern in the resist film. This pattern can be used to form integrated circuit devices, and is currently a critical component in their manufacture.
In an ideal situation, the exposure tool would only allow the radiation to hit the resist material in the areas of the mask that are clear, thus providing sharp edges for the lines and spaces. However, diffraction patterns are formed at the edges of the clear areas, resulting in partial exposure of the resist in those areas. Certain patents have taken advantage of this phenomenon, such as U.S. Pat. No. 4,568,631 issued to Badami et al. on Feb. 4, 1986 and assigned to the assignee of record for the present invention, which discloses utilizing a positive resist and an additive for image reversal in order to create thin resist lines only in the areas where light intensity has been reduced by diffraction effects. However, this procedure uses a resist with conventional positive and negative tone dissolution responses and requires two separate expose and develop operations to form a resist image from the edge of a reticle image.
As the need for higher and higher levels of integration has arisen in the industry, the need for a larger number of lines and spaces in a given area has increased dramatically. In response, a primary subject of research has been enhancement of the exposure tool and reticle system to enhance the aerial image of the circuit pattern. For example, phase shift reticles, shorter wavelength expose tools, higher numerical aperture expose tools, and tools with selective illumination systems are continuing to be developed to improve the pattern density of integrated circuits. Due to the high cost, poor yield, and difficulty of inspection and repair, phase shift reticles are generally not available for use. Due to the complexity of exposure tool design and construction, it is very expensive to build higher numerical aperture and shorter wavelength expose systems.
In another area of activity, efforts are being made to improve the contrast of the photoresist. However, the basic mechanism of operation of the photoresist continues to be the same; that is, the resists behave as either positive or negative tone systems. It is desirable, therefore, to devise new mechanisms of resist response such that conventional optical lithography can be extended to smaller feature sizes without developing new tools and reticles. Additionally, as these new tools and reticles are eventually developed and implemented, these new resist approaches would remain applicable as a further extension of lithographic capability.
Presently, for high performance devices, the control of the image size on the reticle and the control of image size from one batch of wafers to the next comprise the largest contributors to image size variation on the product. Chip yield at high performance is strongly dependent on the uniformity of the image pattern across the chip and the centering of the image pattern at the correct dimension. These limitations exist currently for all types of lithographic patterning which use a reticle; optical, x-ray, and proximity E-beam, for example. The problem of image uniformity across the reticle is especially acute for lithographic techniques that use 1.times. masks, such as x-ray and proximity E-beam lithography. It is therefore desired to provide a photoresist material that allows very precise image control for the image size, independent of the image size control on the reticle.